Used correct backend error message for related issues

ee/app/assets/javascripts/related_issues/components/related_issues_root.vue

@@ -149,11 +149,11 @@ export default {
             // Close the form on submission
             this.isFormVisible = false;
           })
-          .catch(res => {
+          .catch(({ response }) => {
             this.isSubmitting = false;
             let errorMessage = addRelatedIssueErrorMap[this.issuableType];
-            if (res.data && res.data.message) {
-              errorMessage = res.data.message;
+            if (response && response.data && response.data.message) {
+              errorMessage = response.data.message;
             }
             Flash(errorMessage);
           });

ee/spec/javascripts/issuable/related_issues/components/related_issues_root_spec.js

@@ -184,6 +184,22 @@ describe('RelatedIssuesRoot', () => {
           done();
         });
       });
+
+      it('displays a message from the backend upon error', done => {
+        const input = '#123';
+        const message = 'error';
+
+        mock.onAny().reply(409, { message });
+        document.body.innerHTML += '<div class="flash-container"></div>';
+
+        vm.onPendingFormSubmit(input);
+
+        setTimeout(() => {
+          expect(document.querySelector('.flash-text').innerText.trim()).toContain(message);
+          document.querySelector('.flash-container').remove();
+          done();
+        });
+      });
     });
 
     describe('onPendingFormCancel', () => {